Fuel cells provide electricity from chemical oxidation-reduction reactions and possess significant advantages over other forms of power generation in terms of cleanliness and efficiency. Typically, fuel cells employ hydrogen as the fuel and oxygen as the oxidizing agent. The power generation is proportional to the consumption rate of the reactants.
A significant disadvantage that inhibits the wider use of fuel cells is the lack of a widespread hydrogen infrastructure. Hydrogen has a relatively low volumetric energy density and is more difficult to store and transport than the hydrocarbon fuels currently used in most power generation systems. One way to overcome this difficulty is the use of reformers to convert the hydrocarbons to a hydrogen-rich gas stream that can be used Asia feed for fuel cells.
Hydrocarbon-based fuels, such as natural gas, LPG, gasoline, diesel, and fuel oil require conversion processes to be used as fuel sources for most fuel cells. One current art process uses a multi-step process combining an initial conversion process with several clean-up processes. The initial process is most often steam reforming (SR), autothermal reforming (ATR), catalytic partial oxidation (CPOX), or noncatalytic partial oxidation (POX). The clean-up processes are usually comprised of a combination of desulfurization, high temperature water-gas shift, low temperature water-gas shift, selective CO oxidation, or selective CO methanation. Alternative processes include hydrogen selective membrane reactors and filters; however, these have not seen wide application due to expense and technical problems with the technology.
Plasma fuel converters such as plasmatrons reform hydrocarbons to produce a hydrogen-rich gas. DC arc plasmatrons have received particular attention in the prior art. See, for example, U.S. Pat. Nos. 5,425,332 and 5,437,250. DC arc plasmatrons typically operate at low voltage and high current. By operating at high currents and lower voltages, the arc current is high enough that precautions are required to minimize electrode erosion and even melting. High flow rates of cooling water are required to keep the erosion in check. Air flow is required to simultaneously center the discharge on the cathode tip (made of hafnium or other specialized material, embedded in a copper plug) and to move the root of the arc at the anode to minimize erosion at the anode. A constriction is also required to increase the impedance of the discharge (i.e., to operate at high voltages and lower currents than free-flowing arcs). The air flows and the constriction are likely to require operation at elevated pressure (as much as 0.5 bar above ambient pressure), and thus a compressor is likely to be required. Even with these precautions, it is often difficult to extend the lifetime of the electrodes.
Plasmatrons also require relatively sophisticated power supplies for stabilization of the arc discharge. Further, plasmatrons have a limited capability for low power operation. In some reforming applications, the minimum operating power can be significantly greater than needed resulting in unnecessary power loss. DC arc plasmatrons are typically operated at power levels of one kilowatt or more.
Despite the above work, there remains a need for a simple unit for converting a hydrocarbon fuel to a hydrogen-rich gas stream for use with a fuel cell. In particular it is desirable to have a plasma-based fuel reformer that does not require a compressor or a sophisticated power supply for stabilization of the arc discharge. It is also desirable to have a plasma-based fuel reformer having longer electrode life and with a capability of lower power operation when lower flow rates of hydrogen-rich gas are required.
The present invention relates to a plasma-based fuel reformer in which a fuel/air mixture is subjected to conditions including an electrical plasma arc that reforms the fuel/air mixture into a hydrogen-rich gas.
One illustrative embodiment of the present invention is a reactor for carrying out a plasma-based fuel reforming reaction of a fuel/air mixture. In an illustrative embodiment, the reactor includes: a first electrode, a second electrode, and insulator positioned substantially between the first and second electrode except in the area of the reaction chamber; means for providing a fuel/air mixture, and means for providing sufficient voltage to the first and second electrode so as to create a plasma arc between the first and second electrode and that reforms the fuel/air mixture into a hydrogen-rich gas. A preferred illustrative embodiment of the present invention includes a first electrode, in which the first electrode has an inner and outer wall and a longitudinal axis, and in which the inner wall defining a reaction chamber for carrying out the plasma-based fuel/air reforming reaction. The second electrode in such an illustrative embodiment is axially aligned with the longitudinal axis of the first electrode and positioned in the reaction chamber such that when an electrical voltage is applied to the first and second electrodes a plasma arc forms between the first and the second electrode in the reaction chamber. In order to prevent the formation of the plasma arc in a location other than in the reaction chamber, the illustrative embodiment includes an insulator which is positioned between the first and the second electrode so as to prevent the formation of a plasma arc between the first and the second electrodes.
Means for providing a fuel/air mixture to the reaction chamber is also included in the present illustrative embodiment. In one preferred and illustrative embodiment the means for providing a fuel/air mixture to the reaction chamber includes a fuel line axially aligned with the second electrode, the fuel line having a first end and a second end, the first end being in fluid communication with a fuel source and the second end being in fluid communication with a fuel injector, the fuel injector being positioned to inject fuel into the reaction chamber.
Means for diverting the air flow into the reaction chamber is also included in the present illustrative embodiment so as to create a vortex-like flow of air in the reaction chamber. In one illustrative embodiment the means for diverting the airflow into the reaction chamber includes an air flow diverter. The airflow diverter is in fluid communication with a source of air and positioned such that air provided to the airflow diverter by the air source is diverted into the reaction chamber in a vortex-like manner. In one particularly preferred illustrative embodiment, the airflow diverter is helical and is axially aligned with the longitudinal axis of the first electrode.
One illustrative embodiment of the plasma-based fuel reformer disclosed herein includes an exhaust manifold that forms the second end of the first electrode. The exhaust manifold is in fluid communication with the reaction chamber and is positioned so as to direct the flow of hydrogen-rich gas to an exhaust outlet. The exhaust manifold is preferably designed in a manner that promotes the fuel reforming reaction by controlling the pressure of gases entering and exiting the reaction chamber.
As is the case in all of the above illustrative embodiments, it is preferred that the fuel and air mixture provided to the plasma-based fuel reformer is reacted in the reaction chamber so as to form a hydrogen-rich gas. This hydrogen-rich gas may then be used in a wide variety of applications including use as the feed gas for a fuel cell, feed gas for a purification reactor so as to provide substantially pure hydrogen gas, or other suitable applications.
When in use, the plasma-based fuel reformers of the present invention require an electrical power source that can supply sufficient voltage to the first and second electrodes so as to cause the formation of a plasma arc between the two. The voltage necessary to cause the formation of a plasma arc between the first and second electrode depends upon many variables including the spacing of the gap between the two, the relative configuration of the two electrodes and the materials from which the two electrodes are made. In any case, the plasma arc should be sufficient to carry out the fuel/air reformation reaction and the formation of the hydrogen-rich gas. In one preferred and illustrative embodiment, the voltage provided is in the range of approximately 100 volts to about 40 kilovolts and current in the range of approximately 10 milliamperes to about one ampere is sufficient to generate a plasma arc that reforms the fuel/air mixture into the hydrogen-rich gas.
The plasma-based fuel reformer of the present invention may be used in a number of applications, including use as an anode tail gas oxidizer for a fuel cell or as the primary fuel reformer in a hydrogen-rich gas generation system. Other illustrative uses of the plasma-based fuel reformer of the present invention should be apparent to one of skill in the art.